Basically a heat sink is a heat-dissipating device comprising an array of elongated or pin fins affixed to one side of a base plate. Affixed to the other side of the base plate is a heat source such as a computer chip. The heat generated by the heat source is removed from the base plate by a fluid that is either stagnant or flowing over the base plate through the fin array. The stagnant fluid removes heat from the fins and the base plate by natural convection whereas the flowing fluid removes the heat by forced convection. Since the heat transfer rate is higher in forced convection, a flowing fluid is preferable to a stagnant fluid. A fan or a pump provides the flow of the cooling fluid through the heat sink. The extended surface provided by the fins protruding from the base plate aids greatly in dissipating heat to the circumambient stagnant or the flowing fluid.
Intense research is in progress to develop high performance heat sinks for high power electronic devices with heat flux in the range of 100 to 200 W/cm2. Currently available heat sinks are designed to dissipate heat from relatively low power electronic devices with heat flux of about 25 W/cm2 directly into air. Since the heat capacity of air is quite low the currently available heat sinks are not suitable for cooling high power electronic devices. Therefore, in recent years attention has turned to high performance heat sinks entailing the use of the high heat capacity fluids including single-phase liquids as well as fluids capable of undergoing liquid-to-vapor transformation as well as the reverse transformation from vapor-to-liquid. Such high performance heat sinks call for closely spaced fins of optimum dimensions including thickness, height and length
Fabrication of high performance heat sinks poses many challenges dictated by the following considerations:    1. The fabrication process should not lead to distortion of the base plate to which a heat source, like a computer chip, is affixed since the base plate distortion could lead to delamination of the heat source from the base plate in service.    2. The fabrication process should be such that it lends itself for bonding fins of the optimum dimensions including thickness, height, length and spacing. The conventional fabrication processes like machining, extrusion and forging have their limitations and are incapable of producing heat sinks with relatively thin fins for optimal performance.    3. The fabrication process should yield strong and adherent bond between the fin and the base plate so as to minimize the contact resistance between the two thereby ensuring high performance of the heat sink.    4. The fabrication process should be fast and cost effective.
Taking cognizance of the foregoing requirements, a new method of fabricating high performance heat sinks for electronics cooling is developed based on the use of a so called “cold” (near room temperature) isostatic pressing processes which has not, so far as is known, been used for the fabrication of metal heat sinks with the type of fins described above. Cold isostatic processing (CIP) has been used to form non metallic heat sinks. For example, in U.S. Pat. No. 6,538,892, individual graphite disks are die pressed or isostatically pressed, stacked in a spaced array and then heat cured together. Likewise, in published US application 2002/0142065, an integral block of graphite material is isostatically formed, and then later machined into an array of elongated fins. U.S. Pat. No. 6,475,429 discloses a detailed process for CIP processing a heat sink of a particular copper and molybdenum powder mixture, but the heat sink structure itself is basically a solid plate, with a shallow central recess, and no projecting fins. While it discloses no means of forming a fin array, the patent does provide a good, basic explanation of the CIP process.
In the cold isostatic pressing process, uniform hydrostatic pressure is applied to a work piece, typically a charge of powdered metal pre formed substantially to the final shape desired, to compact the powder charge into a suitably solid, void free final shape. In the cold or “CIP” variant of the isostatic process, this is done at near room temperature. Pressure is typically applied to the work piece through the medium of a pressure transmitting elastomeric coating or “sleeve” surrounding the work piece, which, in turn, is subjected to the pressure of a surrounding, high pressure bath of hydrostatic fluid. The fluid acts omni-directionally on and through the sleeve, which stretches and gives to apply the pressure of the fluid to all the exposed surfaces of the part, while preventing the fluid from reaching the interfaces in the part being formed and compressed. The pressure is sufficient to exceed the yield strength of the work piece material, creating a plastic deformation and thorough, solid compression.
Typically, as noted, the isostatic process is applied to a powder charge to create a substantially solid, compact and void free structure, such as a plate or gear. However, a variant of the process has been used, in one known application, to mechanically bond one solid metal part to another. In U.S. Pat. No. 4,627,864, a so called discharge wall for forming a multiplicity of fibers from molten glass consists of a series of small cylindrical metal eyelets bonded in and through close fitting apertures in a metal plate. The eyelets serve as the dies for the fibers being formed. One end of each eyelet is flanged and basically flush to a first side of the plate, while the other end extends out of the aperture and above the second side of the plate. In order to manufacture the apparatus without having to individually weld each eyelet in its aperture, the patented method allows a slight variation of the standard CIP process to be used to bond all the eyelets into the plate at once. To protect the protruding ends of the eyelets during the process, a protective cover of rigid, non-compressible material is placed over them and against the second side of the plate. Then, the usual rubber-sealing sheath is placed around the entire unit, including the rigid, protective cover, evacuated, and subjected to the usual hydrostatic bath. The rubber sheath stretches and deforms into the eyelet interiors, deforming them radially outwardly and tightly into the plate apertures, while keeping the fluid completely sealed away from the part interfaces. Concurrently, the protruding ends of the eyelets remain intact, protected by the rigid cover, which is later simply lifted up and off after the sheath is removed. This process cannot be directly applied to the type of heat sink involved here, for reasons described below.
While the CIP process has not found application in the fabrication of the type of finned heat sinks described above, so far as is known, almost every other imaginable mechanical joining process has been suggested, in addition to the integral machining and one shot molding processes already described. The table below lists a sampling:
U.S. Pat. No.DateInventor(s)TitleRemarks5,523,049June 1996Terpstra et al.Heat Sink and MethodHeat sink comprising a pluralityof Fabricatingof pin fins made by molding athermoplastic material filledwith thermally conductiveparticles and sintering the same5,583,317December 1996Mennucci etMultilayer LaminateHeat sink comprising thermallyal.Heat Sink Assemblybonded flat fins formed out of alaminate having a first layer ofoxygen-free copper joined to asecond layer of oxygen-richcopper6,000,132December 1999ButlerMethod of FormingHeat sink with a base plateHeat Dissipating Finscomprising a plurality of slotsand rectangular fins swaged intothe said slots6,009,937January 2000Gonner et al.Cooling Device forBase plate with rectangular studsElectrical or Electronicto accommodate U-shaped finsComponents . . .6,134,783October 2000Bergman et al.Heat Sink and ProcessAir cooled heat sink comprisingof Manufacturea plurality of extruded tubularfins affixed to a base plate withholes to provide airflowtherethrough6,135,200October 2000Okochi et al.Heat GeneratingAir cooled heat sink withElement Cooling Unitconvoluted louvered fins towith Louversdirect airflow6,199,627March 2001WangHeat SinkHeat sink comprising a pluralityof plate fins with slots andshorter spacer plates alternatelyarranged and held together bymechanical fasters extendingtherethrough6,230,789May 2001Pei et al.Heat DissipatingBase plate with cylindrical studsDevice and Method ofto accommodate an array of U-Making the Sameshaped fins with mounting holesin the flat fin crests6,241,006June 2001ShihHeat Sink for CPUAir cooled heat sink with twofolded fin arrays securedmechanically to the base plate6,260,610July 2001Biber et al.Convoluted Fin HeatHeat sink comprising a foldedSinks with Basefin array with a plurality of U-Topography forshaped fins nested in theThermal Enhancementrectangular grooves withrounded corners and securedthereto by means of a bondingagent6,373,699April 2002ChenHeat DissipationAir cooled heat sink withDeviceextruded chassis and folded finarray secured mechanically tothe chassis
The attached drawing Figures referred to immediately below further describe the features of the type of heat sink referred to above, and further highlight the shortcomings of the known fabrication techniques disclosed in the patents referred to above. FIG. 1 shows a prior art heat sink 1 comprising an array of parallel fins 2 formed integrally with the base plate 3 by machining operation such as electron discharge machining. As already noted, the advantage of heat sink 1 is that the contact resistance between the fin and the base plate is zero since the fin is integrally formed out of the base plate. However, there are many drawbacks of heat sink 1. It is expensive and heavy due to limitations of the machining operation to produce thin fins. As noted, similar drawbacks apply to molding and extrusion processes.
FIG. 2 shows another prior art heat sink 4 comprising a base plate 5 with a plurality of rectangular grooves 6 and a plurality of parallel fins 7 of rectangular profile inserted into the said grooves 6. A shortcoming of this heat sink is that the process of inserting individual fins 7 into the grooves 6 is time-consuming and costly. Moreover, this method of fabrication does not allow fins of the optimal thickness to be used since thin fins of optimal thickness are not easy to insert in the grooves without damage. Another drawback of this heat sink is that the contiguous surfaces of the fin 7 and the groove 6 are not in intimate contact, as there tends to be an inherent gap 8 between the fin 7 and the groove 6. The gap 8 increases the contact resistance between the fin 7 and the base plate 5 thereby undermining the thermal performance of the fins. Use of a fin stock with braze coating can eliminate the gap 8 through in-situ melting of the braze coating in a furnace. However, this process leads to distortion of the heat sink and produces oxide layers thereon, which are not desirable for subsequent bonding of the heat source to the base plate.
FIG. 3 shows another prior art heat sink 9 comprising a base plate 10 with a plurality of rectangular grooves 11 and a U-shaped folded fin 12 nested in the said grooves 11. The U-shaped fins 12 can be made of desired thickness and are easier to insert in the wider grooves 11 as the inserting tool can easily pass between the adjoining vertical protrusions of the U-shaped fins. However, with this heat sink also the contiguous surfaces 13 and 14 of the fin 12 and the groove 11 are not in intimate contact. This gives rise to a gap 15 between the contiguous surfaces undermining the thermal performance of the fins due to presence of the contact resistance. Use of a fin stock with braze coating can eliminate the gap 15 through in-situ melting of the braze coating in a furnace. However, this process leads to distortion of the heat sink and produces oxide layers thereon, which are not desirable for subsequent bonding of the heat source to the base plate.
FIG. 4 shows a prior art heat sink 16 comprising a base plate 17 with an array of studs 18 protruding from the base plate 17 and a U-shaped folded fin 19 with mounting holes stamped in the flat crests of the U-shaped folds to enable press fitting of the fin 19 into the said studs 18 in the base plate 17. Another variation of this heat sink is to eliminate the studs 18 and use rivets to secure the fin 19 to the flat base plate 17. This heat sink also suffers from the drawback of the inherent contact resistance due to the gap between the fin 19 and the base plate 17. Another variation of this heat sink is to use an adhesive coating between the contacting surfaces of the fin 19 and the base plate 17. While the adhesive coating eliminates the gap between the fin and the base plate, it adds its own thermal resistance, which undermines the heat sink performance. Moreover, the effectiveness of the adhesive coating to maintain the bond strength diminishes in time and this could lead to delamination of the fins in service and consequent deterioration in the heat sink performance.
The obvious, straightforward application of the isostatic pressing process described above would be completely ineffective in joining fins like those shown in FIGS. 2 and 3 to a base plate. If an elastomeric sleeve were wrapped over the entire assembly of fins and base plate, and then subjected to a surrounding bath of pressurized fluid, the fin array would be crushed and deformed. If the pre assembled array of fins and base were directly immersed in the pressurized fluid bath, without a surrounding sealing sleeve, fluid would readily enter the interface between the fins and the base, preventing any fin to base joining action from occurring. Nor could one “protect” the fins from the crushing effect of the surrounding sleeve with a rigid cover, in the manner disclosed in U.S. Pat. No. 4,627,864, because such a cover placed over and shielding the fins would also inevitably prevent the compressive action of the fluid deformed sleeve from reaching the very fin to plate interfaces that would need to be compressed.